You can use whatever you want, some guys use the cheapo 4 inch finders and some use 9+ inch models with all the bells and whistles. What you choose to go with comes down to what you want and how much you are willing to spend.
Options to research/consider are
GPS (do you want GPS so you can mark waypoints and use maps etc)
Maps (many fish finders come with map packages, Navionics + are the best commercially available and most common)
Map Creation (some of the new fish finders are capable of making their own depth charts)
Down Imaging (gives a more detailed view of the bottom to help distinguish if sonar return is a rock, tree, fish etc)
Side Imaging (like down imaging but shoots out to each side to help you see even more of the area around you and help you find structure)
I personally like to have GPS and having maps is nice too(Navionics not all that accurate though which makes map creation really nice). My recommendation for a cheaper unit would be something like a Helix 5 GPS G2 with Nav+(costs $500) which has GPS, Navionics maps and can be used to make your own maps using what they call autochart live, there is another similar model with DI as well for a bit more($575). Lowrance has a similar unit in the new Hook2 5 Splitshot with Nav which also has down imaging, you have to pay a yearly subscription to use their live map creation function though, price is $420.
Next steps up are going to something with a 7+ inch screen which you can do if you just want something a bit bigger/more visible. Same models just larger size is what I would look at.
If you want side imaging then get at least a 7 inch model. Helix 7, Hook2 7 or Elite Ti 7 are the cheaper options. Larger units make it even easier to read screen but add to price substantially.
As for mounting non side imaging units will be easier since they use smaller transducers. Some fishing kayaks have systems for mounting transducers/fish finders easier. Your options are to use a transducer arm(works decent but adds drag), mounting transducer to mounting system(if your kayak has one), mounting transducer inside hull to shoot thru the plastic hull(works decent if done right, temp isn't accurate and if poorly done reading quality can be effected), thru scupper hole mount(works on some kayaks). Best way to mount will depend on your kayak and fish finder chosen.
Other accessories worth having are
Anchor trolley(makes boat control when anchored easier)
Crate for tackle and to hold spare rods etc(I am a Hobie guy so use Hobie's H-Crate, really helps keep you organized)
Rod holders (self explanatory, want ones behind you for extra rods and one in front for trolling with)
Most other things are personal preference.
My rig (Hobie Outback with Lowrance Elite Ti 7, transducer mounted in transducer mounting system, H-Crate)
Fishing Kayak questions
Linear Mode
